Referring now to FIG. 1, a data link layer 10 of the open systems interconnection (OSI) model includes a logical link control (LLC) layer 12 and a media access control (MAC) layer 14. The LLC layer 12 addresses and exchanges data with a network layer 16. The MAC layer 14 provides an interface between the LLC layer 12 and a physical layer device 18. The MAC layer 14 frames data for transmission over the network and then passes the frame to the physical layer device 18 for transmission as a stream of bits. In other words, the MAC layer 14 frames data into distinct units or packets that are transmitted one at a time over the network.
The physical layer device 18 typically includes a physical coding sublayer (PCS) 20, a physical medium attachment (PMA) sublayer 22, and an autonegotiation sublayer 24. A medium dependent interface (MDI) 26 connects the physical layer device 18 to media 28 such as twisted pair wires, optical fiber, or other media. The IEEE 802.3 specification, which is hereby incorporated by reference in its entirety, further defines how physical network interfaces operate with different types of media such as coaxial cable, twisted-pair cable, and optical fiber.
The autonegotiation sublayer 24 initiates the exchange of information between two connected network devices and automatically configures the devices to take maximum advantage of their respective abilities. The autonegotiation sublayer 24 advertises the abilities of the network device, acknowledges receipt, identifies common modes of operation, and rejects the use of operational modes that are not shared or supported by both devices. When more than one common mode of operation exists between the devices, an arbitration function of the autonegotiation layer 24 identifies and selects a single mode of operation. After autonegotiation is complete, the devices establish a link and exchange data.
To improve flexibility, the physical layer device 18 of some network devices has been designed to be connected to different types or speeds of media. Alternatively, multiple physical layer devices may be present to handle different types or speeds of media. The manufacturer and/or the user may not know a priori the types or speeds of media that will be used, and the media used may change over time.